Solid state image sensor using an intermediate refractive index antireflection film and method for fabricating the same

ABSTRACT

In a solid state image sensor comprising a plurality of photoelectric conversion regions and a plurality of transfer regions which are formed in a principal surface of a semiconductor substrate, and a plurality of transfer electrodes formed above the transfer regions, a first insulating film, an antireflection film and a second insulating film are formed in the named order on the photoelectric conversion regions. The antireflection film has a refractive index larger than that of the second insulating film but smaller than that of the semiconductor substrate. The stacked film composed of the first insulating film, the antireflection film and the second insulating film, is formed, in the transfer regions, to extend over the transfer electrode which is formed a third insulating film formed on the semiconductor substrate. Preferably, an opening is formed to penetrate through the antireflection film, at a position above the transfer electrode, and after the second insulating film is formed, a sintering is carried out in a hydrogen atmosphere.

This application is a Divisional Application of allowed Application Ser.No. 09/339,683, filed on Jun. 24, 1999, now U.S. Pat. No. 6,060,732.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid state image sensor and a methodfor fabricating the same.

2. Description of Related Art

A CCD (charge coupled device) type solid state image sensor is soconstructed that light is incident on an n-type semiconductor regionformed in a surface of a P-type silicon substrate and an image signal isobtained from a signal charge in the -type semiconductor region.

Referring to FIG. 17, there is shown a sectional view of one example ofthe prior art solid state image sensor, which includes transferelectrodes 3 formed on a P-type silicon substrate 11 with a sixthinsulating film 12 f formed of a silicon oxide film being interposedbetween the transfer electrodes 3 and the P-type silicon substrate 11.In a surface of the substrate 11 between each pair of transferelectrodes 3, an n-type semiconductor region 17 is formed to constitutea photoelectric conversion region. Above this n-type semiconductorregion 17, an aperture is formed in a light block film 16 formed ofaluminum or tungsten. A passivation film 18 is formed to cover the lightblock film 16. Furthermore, on the surface of the substrate It under thetransfer electrode 3, a second n-type semiconductor region 21 is formedto constitute a transfer region, and one end of the second n-typesemiconductor region 21 is separated from the n-type semiconductorregion 17. A p⁺ semiconductor region 26 is formed between the other endof the second n-type semiconductor region 21 and the n-typesemiconductor region 17 in order to isolate pixels from one another. Inthe following, this prior art will be called a first prior art.

In the structure of the first prior art shown in FIG. 17, however, theloss of an incident light is large because of reflection at the surfaceof the P-type silicon substrate 11, and therefore, a satisfactorysensitivity cannot be obtained.

In order to overcome this problem, for example, Japanese PatentApplication Pre-examination Publication No. JP-A-04-206571 (an Englishabstract of which is available and the content of the English abstractis incorporated by reference in its entirety into this application)proposes to form an antireflection film in the photoelectric conversionregion. In the following, the prior art typified by JP-A-04-206571 willbe called a second prior art.

Now, the second prior art will be described with reference to FIG. 18.In FIG. 18, elements corresponding to those shown in FIG. 17 are giventhe same reference numbers.

In this second prior art, for example, an n-type semiconductor region 17becoming a photoelectric conversion region for obtaining a signalcharge, and a second n-type semiconductor region 21 becoming a transferregion for transferring the signal charge read out from the n-typesemiconductor region 17, are formed in a principal surface of the P-typesilicon substrate 11. The n-type semiconductor region 17 and the secondn-type semiconductor region 18 are formed by for example an impuritydiffusion. Incidentally, pixels are isolated from one another by a p⁺semiconductor region 26.

Furthermore, a seventh insulating film 12 g formed of a silicon oxidefilm is formed on the P-type silicon substrate 11. On the silicon oxidefilm 12 g, there is formed an antireflection film 15 formed of a siliconnitride film having a refractive index larger than that of silicon oxidebut smaller than that of silicon. The refractive index of the siliconoxide is about 1.45, and the refractive index of the silicon nitride isabout 2.0. Film thicknesses of the seventh insulating film 12 g and theantireflection film 15 are not greater than 600 Å, respectively, andpreferably on the order of 250 Å to 350 Å, respectively.

By setting the film thicknesses, an antireflection film having arelatively flat spectral characteristics in a visible light region canbe obtained. Thus, by setting the film thicknesses of the seventhinsulating film 12 g and the antireflection film 15 to an appropriatethickness, the reflection factor is suppressed to 12% to 13% at average.Since the incident light was reflected about 40% in the prior art P-typesilicon substrate, the reflection factor can be reduced to about onethird.

A polysilicon layer functioning as a transfer electrode 3 is formedthrough the sixth insulating film 12 f on the silicon oxide film 12 gand the antireflection film 15 above the transfer region. The transferelectrode 3 is coated with an eighth insulating film 12 h formed of asilicon oxide film, and furthermore, is coated with the light block film16 in order to block the incident light. The light block film 16 isformed of for example aluminum. An aperture is formed on the light blockfilm 16 positioned above the n-type semiconductor region 17 so that thelight block film 16 faces onto the n-type semiconductor region 17 in theaperture. The light block film 16 is overcoated with a passivation film18. With this arrangement, a high sensitivity can be realized.

However, the above mentioned structure has the following problems:

A method for effectively reducing a dark current in the solid stateimage sensor is to diffuse hydrogen, as disclosed in for exampleJapanese Patent Application Pre-examination Publication No.JP-A-06-209100 (an English abstract of which is available and thecontent of the English abstract is incorporated by reference in itsentirety into this application).

In the structure in accordance with the second prior art, it is notpossible to sufficiently perform the terminating of dangling bonds at asilicon interface by hydrogen in a final sintering step, which iseffective in reducing the dark current. The reason for this is asfollows: When the sintering is executed after the antireflection film ofthe silicon nitride film is formed, hydrogen is prevented from reachingthe silicon interface by action of the silicon nitride film of theantireflection film.

Incidentally, the solid state image sensor disclosed in JP-A-06-209100has no antireflection film, and JP-A-06-209100 does not disclose amethod for reducing the dark current when the antireflection film isprovided.

A second problem is that since a driving characteristics of the transferregion is limited, it becomes difficult to lower a driving voltage ofthe transfer electrode. In order to increase the sensitivity in thevisible light region, it is necessary to form the antireflection filmhaving the film thickness on the order of 300 Å to 500 Å. If theantireflection film of this film thickness is actually formed on thewhole surface, the film thickness

is the same between the photoelectric conversion region and the transferregion, and therefore, the film thickness in the transfer region is alsoon the order of 300 Å to 500 Å. On the other hand, in order to drive thetransfer electrode with a low voltage, it is necessary to make thecapacitance directly under the transfer electrode as small as possible.For this purpose, it is necessary to make the oxide film directly underthe transfer electrode as thick as possible. Because of this, it isdifficult to lower the driving voltage of the transfer electrode.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a solidstate image sensor having an elevated sensitivity without influencingthe driving characteristics of the transfer electrode, and a method forfabricating the solid state image sensor.

Another object of the present invention is to provide a solid stateimage sensor having an elevated sensitivity and a reduced dark current,without influencing the driving characteristics of the transferelectrode, and a method for fabricating the solid state image sensor.

The above and other objects of the present invention are achieved inaccordance with the present invention by a solid state image sensorcomprising a plurality of photoelectric conversion regions and aplurality of transfer regions which are formed in a principal surface ofa semiconductor substrate, and a plurality of transfer electrodes formedabove the transfer regions, wherein the improvement comprises a firstinsulating film, an antireflection film and a second insulating filmformed in the named order on each of the photoelectric conversionregions, the antireflection film having a refractive index larger thanthat of the second insulating film but smaller than that of thesemiconductor substrate, and the stacked film composed of the firstinsulating film, the antireflection film and the second insulating filmbeing formed, in the transfer regions, to extend over the transferelectrode which is formed on a third insulating film formed on thesemiconductor substrate.

In one embodiment, the antireflection film has an opening formed topenetrate through the antireflection film, at a position above thetransfer electrode.

The first insulating film is formed of a silicon oxide film. Preferably,the first insulating film is formed of a silicon oxide film formed by aLPCVD process. Alternatively, the first insulating film is formed of asilicon oxide film which is formed by a LPCVD process and thenheat-treated at a temperature higher than a growth temperature in theLPCVD process. Here, preferably, the first insulating film has a filmthickness of not greater than 500 Å.

In addition, the antireflection film is formed of a material selectedfrom the group consisting of silicon nitride, tantalum oxide andtitanium oxide strontium. Preferably, the antireflection film is formedof silicon nitride formed by a plasma CVD process. Furthermore, thethird insulating film is formed of a multilayer film selected from thegroup consisting of a silicon oxide film-silicon nitride film-siliconoxide film and a silicon oxide film-silicon nitride film.

According to another aspect of the present invention, there is provideda method for fabricating a solid state image sensor, comprising thesteps of forming a plurality of photoelectric conversion regions and aplurality of transfer regions in a principal surface of a semiconductorsubstrate, forming a plurality of transfer electrodes above the transferthrough a third insulating film, forming a first insulating film overthe whole surface including the photoelectric conversion regions and thetransfer electrodes, forming on the first insulating film anantireflection film having a refractive index smaller than that of thesemiconductor substrate, and forming on the antireflection film a secondinsulating film having a refractive index smaller than that of theantireflection film.

In one embodiment, after the antireflection film is formed, an openingis formed to penetrate through the antireflection film, at a positionabove the transfer electrode. The first insulating film is formed of asilicon oxide film. Preferably, the first insulating film is formed of asilicon oxide film formed by a LPCVD process. Alternatively, the firstinsulating film is formed of a silicon oxide film which is formed by aLPCVD process and then heat-treated at a temperature higher than agrowth temperature in the LPCVD process. Here, preferably, the firstinsulating film has a film thickness of not greater than 500 Å.

In another embodiment, the antireflection film is formed of a materialselected from the group consisting of silicon nitride, tantalum oxideand titanium oxide strontium. Preferably, the antireflection film isformed of silicon nitride formed by a plasma CVD process. On the otherhand, the third insulating film is formed of a multilayer film selectedfrom the group consisting of a silicon oxide film-silicon nitridefilm-silicon oxide film and a silicon oxide film-silicon nitride film.Furthermore, preferably, after the second insulating film is formed, asintering is carried out in a hydrogen atmosphere.

The above and other objects, features and advantages of the presentinvention will be apparent from the following description of preferredembodiments of the invention with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a first embodiment of the solid stateimage sensor in accordance with the present invention;

FIG. 2 is a graph illustrating the difference between the solid stateimage sensor in accordance with the present invention and the solidstate image sensor of the prior art;

FIGS. 3 to 8 are sectional views for illustrating a method in accordancewith the present invention for fabricating the first embodiment of thesolid state image sensor;

FIG. 9 is a sectional view of a second embodiment of the solid stateimage sensor in accordance with the present invention;

FIG. 10 is a sectional view of a third embodiment of the solid stateimage sensor in accordance with the present invention;

FIGS. 11 to 16 are sectional views for illustrating a method inaccordance with the present invention for fabricating the thirdembodiment of the solid state image sensor;

FIG. 17 is a sectional view of the solid state image sensor of the firstprior art; and

FIG. 18 is a sectional view of the solid state image sensor of thesecond prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, embodiments of the present invention will be described withreference to the accompanying drawings.

First Embodiment

Referring to FIG. 1, there is shown a sectional view of a firstembodiment of the solid state image sensor in accordance with thepresent invention. To make it easier to compare this embodiment with thesolid state image sensor of the second prior art, the sectional view ofFIG. 1 is similar to that of FIG. 8, and elements similar to those shownin FIG. 18 are given the same reference numbers.

The first embodiment of the solid state image sensor shown in FIG. 1includes a P-type silicon substrate 11. In a principal surface of theP-type silicon substrate 11, a plurality of n-type semiconductor regions17 each becoming a photoelectric conversion region, a plurality ofsecond n-type semiconductor regions 21 each becoming a transfer region,and a plurality of p⁺ semiconductor regions 26 for isolating pixels fromone another, are formed. On the n-type semiconductor region 17 in theP-type silicon substrate 11, a thin silicon oxide film is formed toconstitute a first insulating film 12 a. On this thin silicon oxide film12 a, there is formed an antireflection film 15 formed of a siliconnitride film having a reflective index larger than that of silicon oxidebut smaller than that of silicon. In the transfer region, thisantireflection film 15 is formed above a transfer electrode 3 with thefirst insulating film 12 a of the silicon oxide film being interposedbetween the antireflection film 15 and the transfer electrode 3. Inaddition, the antireflection film 15 formed above the transfer electrode3 is partially removed to form an opening 15 a for supplying hydrogenfrom an external.

In the transfer region, on the silicon substrate 11 there is formed aso-called ONO film which is formed by depositing a silicon oxide film 12c (third insulating film) on a silicon interface, and by depositing asilicon nitride film 24 (second silicon nitride film) on the siliconoxide film 12 c, and then by depositing a silicon oxide film 12 d(fourth insulating film) on the second silicon nitride film 24. On thisONO film, the transfer electrode 3 is formed. Thus, the antireflectionfilm 15 formed of the silicon nitride film and the second siliconnitride film 24 are separated from each other by the first insulatingfilm 12 a, so that the silicon nitride film 15 and the second siliconnitride film 24 are in no contact with each other.

The antireflection film 15 is coated with a second insulating film 12 bformed of a silicon oxide film, and further coated with a light blockingfilm 16 in order to block the incident light. This light blocking film16 is formed of tungsten or aluminum. An aperture is formed to penetratethrough the light block film 16 positioned above the n-typesemiconductor region 17 so that the light block film 16 faces onto then-type semiconductor region 17 in the aperture. The light block film 16is overcoated with a passivation film 18. A planarization layer 22 isformed to cover the passivation film 18, and micro lens 23 are formed onthe planarization layer 22, as shown.

With the above mentioned structure, a high sensitivity can be realizedwith giving no adverse influence to a vertical driving characteristics,which was a problem in the prior art. In addition, in the structureshown in FIG. 1, a dark current is greatly reduced if the sintering in ahydrogen atmosphere is carried out after the opening 15 a is formed inthe antireflection film 15 above the transfer region as shown in FIG. 1.However, a detailed explanation of this sintering process and anadvantage obtained by the sintering will be made when a method inaccordance with the present invention for fabricating the firstembodiment of the solid state image sensor will be described later.

Now, why the sensitivity is elevated will be described. Ordinarily,about 30% of a visible light incident on a silicon-oxide interface isreflected. This is a cause for lowering the sensitivity. A highreflection factor of the silicon-oxide interface is because a differencein refractive index between the silicon and the silicon oxide is large(the refractive index of the silicon is about 3 to 4, and the refractiveindex of the silicon oxide is about 1.4). In order to make thisreflection factor as small as possible, a film having the refractiveindex larger than of the silicon oxide but smaller than that of siliconis used as the antireflection film. As a result, an incidence efficiencyto the silicon can be elevated, so that the sensitivity can be improved.

The result of an experiment of the spectral sensitivity is shown in FIG.2. It would be understood that by forming the antireflection film, theincidence efficiency to the silicon is elevated, and the sensitivity iselevated about 23%.

This first embodiment is characterized in that no antireflection film isformed between the transfer electrode and the silicon substrate.Therefore, it is possible to design the film thickness of the siliconoxide film under the transfer electrode and the film thickness of thesilicon oxide film under the antireflection film in the photoelectricconversion region, independently of each other. Accordingly, the drivingcharacteristics of the transfer electrode is not restricted at all bythe film thickness of the antireflection film.

Now, a method in accordance with the present invention for fabricatingthe first embodiment of the solid state image sensor will be describedwith reference to FIGS. 3 to 8.

FIG. 3 shows a condition that the n-type semiconductor regions 17, thesecond n-type semiconductor regions 21 and the p+ semiconductor regions26 are formed in a principal surface of the semiconductor substrate 11,and the ONO film composed of the third silicon oxide film 12 c, thesecond silicon nitride film 24 and the fourth silicon oxide film 12 d,are formed on the principal surface of the semiconductor substrate 11,and furthermore, a polysilicon layer 3 becoming the transfer electrodeare formed on the ONO film. The film thicknesses of the ONO film and thepolysilicon layer 3 are as follows: The silicon oxide film 12 c formedon the principal surface of the semiconductor substrate 11 has a filmthickness of 400 Å, and the second silicon nitride film 24 formed on thesilicon oxide film 12 c has a film thickness of 200 Å. The fourthsilicon oxide film 12 d formed on the second silicon nitride film 24 hasa film thickness of 200 Å, and the polysilicon layer 3 formed on thefourth silicon oxide film 12 d has a film thickness of 2000 Å.

Thereafter, a resist pattern 25 is formed on the polysilicon layer 3,and then, the polysilicon layer 3 and the ONO film in the photoelectricconversion region are etched by using the resist pattern 25 as a mask,as shown in FIG. 4.

Then, the resist pattern 25 is removed, and the first insulating film 12a formed of the silicon oxide film is deposited on the whole surface. Atthis time, in order to surely separate the second silicon nitride filmand the antireflection film from each other, it is preferred to form asilicon oxide film which has a uniform step coverage and whose filmthickness can be precisely controlled. For this purpose, the firstinsulating film 12 a of the silicon oxide film is formed by a LPCVD (lowpressure chemical vapor deposition) process. Thereafter, a heattreatment may be carried out at a temperature higher than a growthtemperature in the film deposition process. In addition, this filmfunctions as a buffer film for relaxing a stress between the siliconsubstrate surface and the silicon nitride film of the antireflectionfilm. In this regard, the thinner the film thickness is, the higher thesensitivity becomes. In particular, in order to elevate the sensitivityin the visible light range, the film thickness is preferred to be notgreater than 5000 Å.

On whole surface of the silicon oxide film 12 a formed as shown in FIG.5, the antireflection film 15 formed of the silicon nitride film havingthe thickness of about 500 Å is formed. Thereafter, a portion of theantireflection film 15 positioned above the transfer electrode 3 isremoved to form the opening 15 a penetrating through the antireflectionfilm 15, as shown in FIG. 6. Furthermore, the second insulating film 12b formed of the silicon oxide film is deposited to cover theantireflection film 15. After this process, an aluminum film 16 having athickness of 3000 Å to 4000 Å is formed to constitute the light blockfilm.

After the aluminum film 16 is formed, a portion of the aluminum film 16located above the photoelectric conversion region is removed to form theaperture, as shown in FIG. 7.

Thereafter, the passivation film 18 is formed on the whole surface, asshown in FIG. 8, and then, the sintering in the hydrogen atmosphere iscarried out. The hydrogen permeates through the silicon oxide film 12 bwhich is an interlayer film between the light block film 16 and theantireflection film 15, and then enters through the opening 15 a formedin the antireflection film 15 to the silicon oxide film 12 a under theantireflection film 15, and further permeates through the silicon oxidefilm 12 a between the antireflection film 15 and the second siliconnitride film 24, and finally reaches the silicon interface at theprincipal surface of the substrate 11. The hydrogen having reached thesilicon interface terminates dangling bonds at the silicon interface. Asa result, the dark current is reduced.

Thereafter, as shown in FIG. 1, the planarization layer 22 and the microlens 23 are formed. Thus, the first embodiment of the solid state imagesensor shown in FIG. 1 is obtained.

As mentioned above, since the refractive index of the silicon oxide isabout 1.45 and the refractive index of the silicon is about 3 to 4, theantireflection film 15 is formed of the silicon nitride film in thisembodiment (the refractive index of the silicon nitride is about 2.0).

However, the antireflection film 15 can be formed of other materials,for example, tantalum oxide or titanium oxide strontium.

In addition, when the antireflection film 15 is formed of the siliconnitride film, since the silicon nitride film formed by a plasma CVDcontains a large amount of hydrogen in the film, in comparison with thesilicon nitride film formed by the LPCVD, the antireflection film 15 ofthe silicon nitride film formed by the plasma CVD is more effective inreducing the dark current.

Second Embodiment

Referring to FIG. 9, there is shown a sectional view of a secondembodiment of the solid state image sensor in accordance with thepresent invention. In FIG. 9, elements corresponding to those shown inFIG. 1 are given the same reference numbers, and explanation will beomitted for simplification of description.

In the first embodiment shown in FIG. 1, the ONO film is formed on thesilicon interface in the transfer region, but in the second embodimentshown in FIG. 9, only a fifth insulating film 12 e of a silicon oxidefilm is formed on the silicon interface in the transfer region, and thetransfer electrode 3 is formed on the fifth insulating film 12 e. Sincethe ONO film is not formed, the fabricating process can be shortened incomparison with the first embodiment. This is only the differencebetween the first and second embodiments.

Third Embodiment

Referring to FIG. 10, there is shown a sectional view of a thirdembodiment of the solid state image sensor in accordance with thepresent invention. In FIG. 10, elements corresponding to those shown inFIG. 1 are given the same reference numbers, and explanation will beomitted for simplification of description.

As seen from comparison between FIG. 1 and FIG. 10, the third embodimentis different from the first embodiment only in that no opening 15 a isformed in the antireflection film 15 positioned above the transferelectrode 3, and therefore, the antireflection film 15 completely coversan upper surface of the transfer electrode.

With this arrangement, the sensitivity can be elevated with giving noadverse influence to a vertical driving characteristics, which was aproblem in the prior art, similarly to the first embodiment.

In addition, since the antireflection film 15 is formed to completelycover an upper surface of the transfer electrode, namely, since noopening 15 a is formed in the antireflection film 15, the step forpartially removing the antireflection film 15 to form the opening 15 ais no longer necessary. Therefore, the fabricating process can beshortened in comparison with the first embodiment.

On the other hand, the effect for reducing the dark current cannot beexpected. However, as mentioned above, since the silicon nitride filmformed by the plasma CVD contains a large amount of hydrogen in thefilm, if the antireflection film 15 is constituted of the siliconnitride film formed by the plasma CVD, the hydrogen contained in thesilicon nitride film formed by the plasma CVD, terminates dangling bondsat the silicon interface in the sintering process. As a result, the darkcurrent is reduced.

Similarly to the first embodiment, the antireflection film 15 can beformed of not only the silicon nitride film but also other materials,for example, tantalum oxide or titanium oxide strontium. However, whenthe antireflection film 15 is formed of the material such as tantalumoxide or titanium oxide strontium, other than the silicon nitride, theeffect for reducing the dark current cannot be expected.

In this third embodiment, the silicon oxide film 12 a under theantireflection film in the photoelectric conversion region is preferredto be formed to have a film thickness of not greater than 500 Å,similarly to the first embodiment.

Now, a method in accordance with the present invention for fabricatingthe third embodiment of the solid state image sensor, will be describedwith reference to FIGS. 11 to 16.

Since FIGS. 11 to 13 are the same as FIGS. 3 to 5, respectively,explanation will be omitted. Thereafter, as shown in FIG. 14, theantireflection film 15 formed of the silicon nitride film having thethickness of about 500 Å is formed to cover the whole surface of thesilicon oxide film 12 a. Furthermore, the silicon oxide film 12 b isformed to cover the antireflection film 15, and the light block film 16formed of aluminum or tungsten is formed on the silicon oxide film 12 b.

After the light block film 16 is formed, a portion of the light blockfilm 16 located above the photoelectric conversion region is removed toform the aperture, as shown in FIG. 15.

Thereafter, the passivation film 18 is formed on the whole surface, asshown in FIG. 16, and then, as shown in FIG. 10, the planarization layer22 and the micro lens 23 are formed. Thus, the second embodiment of thesolid state image sensor shown in FIG. 1 is obtained.

As seen from the above, according to the present invention, in order toelevate the sensitivity of the solid state image sensor, theantireflection film formed of for example the silicon nitride film,having a refractive index larger than that of silicon oxide but smallerthan that of silicon, is formed on the n-type semiconductor regionconstituting the photoelectric conversion region, and on the other hand,the same antireflection film is formed above the transfer electrode inthe transfer region. Accordingly, the sensitivity can be elevatedwithout influencing the driving characteristics of the transferelectrode.

Furthermore, the portion of the antireflection film above the transferelectrode is removed to form the opening or the antireflection film isformed of the silicon nitride containing a large amount of hydrogen, andthen, the sintering for supplying hydrogen is carried out. Therefore,since the dangling bonds at the silicon interface are terminated byhydrogen, the dark current can be greatly reduced.

The invention has thus been shown and described with reference to thespecific embodiments. However, it should be noted that the presentinvention is in no way limited to the details of the illustratedstructures but changes and modifications may be made within the scope ofthe appended claims.

What is claimed is:
 1. A method for fabricating a solid state image sensor, comprising the steps of forming a plurality of photoelectric conversion regions, a plurality of p+ semiconductor regions, each of said p+ semiconductor regions contacting only one side of one of said photoelectric conversion regions, and a plurality of transfer regions in a principal surface of a semiconductor substrate, forming a plurality of transfer electrodes above said transfer regions through a third insulating film, forming a first insulating film over the whole surface including said photoelectric conversion regions and said transfer electrodes, said first insulating film contacting said transfer electrodes, forming on said first insulating film an antireflection film having a refractive index smaller than that of said semiconductor substrate, and forming on said antireflection film a second insulating film having a refractive index smaller than that of said antireflection film.
 2. A method for fabricating a solid state image sensor, claimed in claim 1 wherein after said antireflection film is formed, an opening is formed to penetrate through said antireflection film, at a position above said transfer electrode.
 3. A method for fabricating a solid state image sensor, claimed in claim 1 wherein said first insulating film is formed of a silicon oxide film.
 4. A method for fabricating a solid state image sensor, claimed in claim 1 wherein said first insulating film is formed of a silicon oxide film formed by a LPCVD process.
 5. A method for fabricating a solid state image sensor, claimed in claim 1 wherein said first insulating film is formed of a silicon oxide film which is formed by a LPCVD process and then heat-treated at a temperature higher than a growth temperature in said LPCVD process.
 6. A method for fabricating a solid state image sensor, claimed in claim 1 wherein said first insulating film has a film thickness of not greater than 500 Å.
 7. A method for fabricating a solid state image sensor, claimed in claim 1 wherein said antireflection film is formed of a material selected from the group consisting of silicon nitride, tantalum oxide and titanium oxide strontium.
 8. A method for fabricating a solid state image sensor, claimed in claim 1 wherein said antireflection film is formed of silicon nitride formed by a plasma CVD process.
 9. A method for fabricating a solid state image sensor, claimed in claim 1 wherein said third insulating film is formed of a multilayer film selected from the group consisting of a silicon oxide film-silicon nitride film-silicon oxide film and a silicon oxide film-silicon nitride film.
 10. A method for fabricating a solid state image sensor, claimed in claim 2 wherein after said second insulating film is formed, a sintering is carried out in a hydrogen atmosphere.
 11. A method for fabricating a solid state image sensor, claimed in claim 8 wherein after said second insulating film is formed, a sintering is carried out in a hydrogen atmosphere.
 12. A method for fabricating a solid state image sensor, comprising the steps of: forming a plurality of photoelectric conversion regions and a plurality of transfer regions in a principal surface of a semiconductor substrate; forming a plurality of transfer electrodes above said transfer regions through a third insulating film; forming a first insulating film over the whole surface including said photoelectric conversion regions and said transfer electrodes; forming on said first insulating film an antireflection film having a refractive index smaller than that of said semiconductor substrate; forming an opening to penetrate through said antireflection film so as to reach said first insulating film, at a position above said transfer electrode; and forming on said antireflection film a second insulating film having a refractive index smaller than that of said antireflection film, so that said opening is filled up with said second insulating film.
 13. A method for fabricating a solid state image sensor, claimed in claim 12 wherein said first insulating film is formed of a silicon oxide film which is formed by a LPCVD process and then heat-treated at a temperature higher than a growth temperature in said LPCVD process.
 14. A method for fabricating a solid state image sensor, claimed in claim 12 wherein said first insulating film has a film thickness of not greater than 500 Å.
 15. A method for fabricating a solid state image sensor, claimed in claim 12 wherein after said second insulating film is formed, a sintering is carried out in a hydrogen atmosphere.
 16. A method for fabricating a solid state image sensor, comprising the steps of: forming a plurality of photoelectric conversion regions and a plurality of transfer regions in a principal surface of a semiconductor substrate; forming a plurality of transfer electrodes above said transfer regions through a third insulating film which is formed of a multilayer film selected from the group consisting of a silicon oxide film-silicon nitride film-silicon oxide film and a silicon oxide film-silicon nitride film; forming, as a first insulating film, a silicon oxide film by a LPCVD process, over the whole surface including said photoelectric conversion regions and said transfer electrodes; forming on said first insulating film an antireflection film which has a refractive index smaller than that of said semiconductor substrate and which is formed of a material selected from the group consisting of silicon nitride, tantalum oxide and titanium oxide strontium; forming an opening to penetrate through said antireflection film so as to reach said first insulating film, at a position above said transfer electrode; and forming on said antireflection film a second insulating film having a refractive index smaller than that of said antireflection film, so that said opening is filled up with said second insulating film.
 17. A method for fabricating a solid state image sensor, claimed in claim 16 wherein after said silicon oxide film of said first insulating film is formed by said LPCVD process, said silicon oxide film of said first insulating film is heat-treated at a temperature higher than a growth temperature in said LPCVD process.
 18. A method for fabricating a solid state image sensor, claimed in claim 16 wherein said first insulating film has a film thickness of not greater than 500 Å.
 19. A method for fabricating a solid state image sensor, claimed in claim 16 wherein said antireflection film is formed of silicon nitride formed by a plasma CVD process.
 20. A method for fabricating a solid state image sensor, claimed in claim 16 wherein after said second insulating film is formed, a sintering is carried out in a hydrogen atmosphere. 